1. Field of the Invention
This invention relates to a motor control circuit for a brushless DC motor, and, more particularly, relates to a stop control circuit, or braking circuit for the DC motor. Specifically, this invention is directed to a circuit which, by purely electrical means, can cause the DC motor to stop precisely.
2. Brief Description of the Prior Art
In a conventional DC brushless motor, either a mechanical brake or an electromechanical brake is used for stopping the motor. These mechanical and electromechanical brakes have a distinct disadvantage in that the braking force applied to the motor varies drastically with variations in the mechanical load and with temperature changes. In addition, as the motor and the associated brake age and wear, the amount of braking force tends to vary.
One arrangement of such conventional motors imposes a mechanical braking load at all times during operation. While such arrangement does permit the motor to be stopped quickly, it has a severe drawback in that a constant braking force has to be overcome to drive the motor, and additional power is consumed in overcoming this force, even when the motor is operated at a constant rotational speed.
Alternatively, a brushless DC motor can be arranged with a mechanical brake that is applied only at stopping. While this has the desirable advantage of eliminating the need for power to overcome any constant braking force, it has the disadvantage in that moving parts, which can wear and break down, are required. In addition, a solenoid plunger or the like is required to drive the braking mechanism. The arrangement including the plunger is somewhat bulky and cumbersome, and must be carefully constructed to be sufficiently reliable, as the plunger must be ready at all times, although it is required only upon braking.
If the brushless DC motor is used, for example, as a capstan motor in a video tape recorder (VTR) used for tape editing, the stopping point of the motor must be determined precisely. Thus, if an electromechanical brake is used, the time of application of the plunger for driving the brake mechanism must be determined with great accuracy. This usually requires precise manual adjustment.
If a mechanical brake is used with such a capstan motor, it is possible that the stop position thereof can be shifted due to shifts in external forces, such as tape back tension. In such case, the motor will tend to stop at an incorrect position. Therefore, these conventional braking systems have the disadvantage that the stop position, even though having been correctly adjusted initially, will tend to vary subsequently.